 At the end of this session students will be able to explain inter-symbol interference. We also expect they should be able to describe properties of i-pattern. They should also explain measurement techniques of inter-symbol interference. So these are the contents that we are going to see throughout this session. Now let us start with introduction of inter-symbol interference. Now we will come to this slide afterwards. First we will take one example. See here this is the message that we are considering suppose 101 is the transmitted pulses. So these are the levels that we see that 0 and 1. So the advantage of the PCM is that information does not lie in any property of the pulse but it lies in the presence or absence of the pulse. So that is the advantage of the PCM system. So here this is the data 101 shown here that this is 0 level and this is level 1. Now when such type of message when it is transmitted through the channel the positive is that the message is going to be distorted because of the noise. So here I have shown two examples the received pulses suppose it is received as it is but you see that the shape of the pulse is little bit changed because of the property of or because of the noise present in the channel or inter-symbol interference. So the shape is changed. Now the function or the role of the receiver is that it is going to identify that the received bit is 1 or 0. How it can be done? Here this is the one and this is the related output that we see at the receiver. Now here within the bit time interval the sample is taken and the sample is compared with certain threshold value and the decision is taken that the received bit is 1 or 0. So here we observe that when the sample is taken it is above the threshold value and the decision taken as the received bit is 1. In the second bit time interval again the sample is taken and it observed that the sample value is lower than the threshold value and the decision taken as the bit is 0. Next here again we observe that the sample value is above the threshold value and the decision taken as 1. Now second the possibility of the receiver has been shown earlier where we observe that the same message is received as in this shape and so when the sample is taken and it is compared with the threshold value we observe that the output is 0 1 1 instead of 1 0 1. So basically what we observe that because of the distortion or the noise the intersymbol interference that observes. So here the briefly what we can say that in telecommunication intersymbol interference is a form of distortion of signaling which one symbol interferes with the subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effects as noise thus making the communication less reliable this is undesired thing. The spreading of the pulse beyond its allotted time interval causes it to interfere with neighboring pulses and ISI is usually caused by multipath propagation or the inherent linear or non-linear frequency response of a communication channel causing successive symbols to blur together. Now what we will see now how this ISI is measured using eye pattern. So again we will see the example first then again we will come back to this slide. So here so this is a what we have shown that this is a simple ideal pulse this is a bit time interval which is inputted or which is transmitted through the channel that is nothing but your transmission medium and the output that we observe which is not of the same shape so it has a rounded shape. So how we can measure it we can measure it using the CRO. As all we know that CRO has got two plates vertical plate and horizontal plate. So for vertical plate the received signal is fed the bit stream whatever the PCM signal output is given to the vertical plate or we can it is a y plate whereas for x plate it is a time based frequency which is adjusted to bit rate. So when we do this the output that you observe on CRO is of the shape of eye pattern as we are going to see that in within this bit time interval the output is set to one bit time interval and whatever the next bits outputs those will be overlapped on each other and overall the shape of the pattern that you see on the CRO is of the eye diagram or eye pattern that is also the reason why it is named as eye diagram. So generally this interest symbol interference occurs because of the two major reasons one is known as multipath propagation the other is band limited channel. What is a multipath propagation that here when we transmit a signal through the air we receive that message to different paths as you know that the reflection and the refraction or other reasons the signal that is receiving at the receiving point. It can get all that messages to different paths and because of this multipath propagation the signal that is received at that point it is going to vary as you know that the signal is reaching at the receiver since it is reaching through different paths with different distance. So some delay is going to observe at the receiving point. So some signal is going to be attained also. So that is also the reason why the shape of the waveform that is changed and the second is the band limited channel as you know that property of the channel is that it is limited to a certain frequency. Every channel you have its own cutoff frequency that means beyond this cutoff frequency the signal are not passed and that information is lost that frequency component that we lose it. So these are the two major reasons that is multipath propagation and the band limited channel the reason because of these two reasons we observe this inter-symbolic interference. So even when we increase the bit rate here I have shown the two say for example one and one suppose these are the two bits and these two bits are separated by some certain bit time interval that generally known as guard time. So when the bit rate is increased definitely what is going to happen that the bit whatever the guard time that is going to be reduced. So these two bits will come closer and closer. So that is also the reason why we observe inter-symbol interval. In telecommunication an eye pattern also known as eye diagram is an oscilloscope display in which a digital signal from a receiver is repetitively sampled and applied to the vertical input while the data rate is used to trigger the horizontal sweep that I have shown in the already we have understood that. It is so called because for several types of coding the pattern looks like a series of eyes between a pair of rails. It is a tool for the evaluation of the combined effect of channel noise and inter-symbol interference on the performance of a base band pulse transmission system. So eye diagrams are quick visual means to quickly identify whether there are any signal integrity issues before moving on to the more refined analysis. Eye diagrams help with effective analysis of serial data and offer both qualitative and quantitative insight regarding the data communications signal path itself and what might be affecting signal integrity. Eye patterns provide a practical and very convenient method assessing the extent of eyes eye degradation. So here if you see if you want to see the different properties of the eye diagram here in this figure there are different numbers have been shown. So this is the zero level. It is a measure of the mean value of the logical zero of an eye diagram. This is one level is a measure of the mean value of the logical one of an eye diagram. Rise time that is what is shown. This is the rise time. It is a measure of the transmission time of the data from 10% level to 90% level on the upward slope of an eye diagram. Fall time that has been shown. This is the fall time is a measure of the transition time of the data from 90% to 10% level on the downward slope of an eye diagram. This is a height eye height. It is a measure of vertical opening of an eye diagram measured from level 0 to level 1. Noise on the eye will cause eye to close. Now there are some other properties. This is eye width. This is eye width measures horizontal opening of an eye measured between crossing points of the eye. Then deterministic jitter is the deviation of a transition from ideal time caused by reflection relative to other transitions. Eye amplitude is the difference between the logic one and logic zero level. Bit rate data rate is inverse of bit period. It is one upon bit period is so these are the other differences.